1. Field of the Invention
The present invention relates to a baking method for heating an object to remove gas molecules occluded in the object. More specifically, the present invention relates to a baking method most suitable for baking a superconducting cable, various pipes such as a cold water pipe, a feed water pipe, an LNG pipe, a refrigerant pipe, a hot water pipe, a hot water supply line, and a heating medium pipe, pipe fittings, and a heat insulation pipe or a heat insulation vessel used as a piping instrument.
2. Description of the Background Art
For production of a vacuum double heat insulation pipe having a vacuum layer formed between an inner pipe and an outer pipe, for example, heat processing called baking to heat the heat insulation pipe is performed to remove gas molecules occluded in a material of the heat insulation pipe to enhance a degree of vacuum. For the baking, roughly two methods are known. One method is heating an object by arranging heating means such as a heater near the heat insulation pipe (for example, see Japanese Patent Laying-Open No. 9-296779 and No. 11-125609). The other method is introducing heated gas into the vacuum layer of the heat insulation pipe and then exhausting the gas (for example, see Japanese Patent Laying-Open No. 9-273659).
The above-mentioned conventional methods, however, cause problems as follows.
<Method of Using Heating Means Such as a Heater>
(1) Energy is highly lost from the heating means.
With a heater, a baking object is heated from the outside of the vacuum layer mainly with conductive heat from the heater. Thus, a member of the baking object which faces the vacuum layer cannot be heated directly, and energy is highly lost from a heat source. Though the member facing the vacuum layer can be heated more efficiently by arranging the heater inside the vacuum layer, it is not practical because the heater and a wiring thereof release gas that decreases a degree of vacuum and, in addition, it complicates the configuration to remove the wiring from the vacuum layer to an atmospheric side.
(2) A baking device becomes so large and complicated that a space for setting the device is limited.
A baking device becomes larger by installation of the heater, which limits a place for setting. In addition, the larger the surface area of the member facing the vacuum layer is, the larger the heater and a power supply thereof become, and the wiring of the heater will also be complicated. In particular, it is not easy to wind the heater around whole length of a long body as a heat insulation pipe. On the other hand, without upsizing the heating means such as a heater, a size of the baking object that can be heated is then limited.
(3) Selection of elements for use in the baking device is limited because portions other than the baking object are also heated.
For portions of the baking device located near the heater are also heated, extra portions other than the baking object are also heated to high temperature (such as 70° C. or higher). Therefore, as prescribed heat resistance is also needed for the elements used in the heated portions, selection of the elements is limited.
(4) It is difficult to heat the whole baking object uniformly.
In the baking method using a heater, heat distribution becomes uneven and uniform baking effect is not achieved unless the heater is evenly mounted for the baking object. In particular, when the baking object is large or long in size and has a shape that has projections and depressions, it is hard to evenly mount the heater for the object.
<Method of Introducing Heated Gas into a Baking Object>
(1) A required amount of gas increases in proportion to a baking time.
Nitrogen gas or the like is used as the heated gas, and thus a cylinder to feed the gas and heating means for the gas are included in the baking device. For a longer baking time, the amount of gas needed increases, and thus a large cylinder is required which leads to upsizing of the device. In addition, as the object is indirectly heated with the heated gas, the heating is not energy-efficient.
(2) It is difficult to heat the whole baking object uniformly.
The heated gas is usually introduced from one end side of the heat insulation pipe and exhausted from the other end side. As temperatures of the heated gas are different at the inlet and outlet of the gas, it is difficult to uniformly heat the baking object. In particular, when the vacuum layer is large and long, the effect is conspicuous and the object is hardly baked to uniform temperature. In addition, because there is a necessity to increase a degree of vacuum (or decrease a pressure) in the vacuum layer of the vacuum heat insulation pipe (vessel), the heated gas cannot be fed again once evacuation is started after the heating of the object with introduced heated gas, even when a temperature in the vacuum layer decreases. Therefore, effect of the baking cannot be exerted efficiently.